linux-kernel - Re: [RFC PATCH] ALSA: pcm: Modify double acknowledged interrupts check condition

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Date:	Tue, 19 May 2015 09:34:47 +0200
From:	Takashi Iwai <tiwai@...e.de>
To:	Koro Chen <koro.chen@...iatek.com>
Cc:	<matthias.bgg@...il.com>, <broonie@...nel.org>, <perex@...ex.cz>,
	<srv_heupstream@...iatek.com>,
	<linux-mediatek@...ts.infradead.org>, <s.hauer@...gutronix.de>,
	<linux-arm-kernel@...ts.infradead.org>,
	<linux-kernel@...r.kernel.org>, <alsa-devel@...a-project.org>
Subject: Re: [RFC PATCH] ALSA: pcm: Modify double acknowledged interrupts check condition

At Wed, 13 May 2015 22:39:03 +0800,
Koro Chen wrote:
> 
> Currently in snd_pcm_update_hw_ptr0 during interrupt,
> we consider there were double acknowledged interrupts when:
> 1. HW reported pointer is smaller than expected, and
> 2. Time from last update time (hdelta) is over half a buffer time.
> 
> However, when HW reported pointer is only a few bytes smaller than
> expected, and when hdelta is just a little larger than half a buffer time
> (e.g. ping-pong buffer), it wrongly treats this IRQ as double acknowledged.
> 
> The condition #2 uses jiffies, but jiffies is not high resolution
> since it is integer. We should consider jiffies inaccuracy.
> 
> Signed-off-by: Koro Chen <koro.chen@...iatek.com>

The condition looks too strict, indeed.  I applied the patch as is
now.

BTW, the similar check is already present for the free-wheeling case.
I'm going to clean up the code a bit for using the common code.


thanks,

Takashi


> ---
>  sound/core/pcm_lib.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/sound/core/pcm_lib.c b/sound/core/pcm_lib.c
> index ac6b33f..7d45645 100644
> --- a/sound/core/pcm_lib.c
> +++ b/sound/core/pcm_lib.c
> @@ -339,7 +339,7 @@ static int snd_pcm_update_hw_ptr0(struct snd_pcm_substream *substream,
>  		if (delta > new_hw_ptr) {
>  			/* check for double acknowledged interrupts */
>  			hdelta = curr_jiffies - runtime->hw_ptr_jiffies;
> -			if (hdelta > runtime->hw_ptr_buffer_jiffies/2) {
> +			if (hdelta > runtime->hw_ptr_buffer_jiffies/2 + 1) {
>  				hw_base += runtime->buffer_size;
>  				if (hw_base >= runtime->boundary) {
>  					hw_base = 0;
> -- 
> 1.8.1.1.dirty
> 
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